The exact thing you describe has been happening to me as well. In my case, I first noticed it around 12:10am GMT on January 8th 2014. I'd played for about half an hour earlier in the day too, at about 4pm on January 7th 2014, and the issue wasn't present then. Since the alpha was released I've never had this issue. I checked just now, and it's still happening.

I was playing with a friend both times and he told me that on his side I was constantly equipping and unequipping an item, and freely moving around the screen like I was regularly playing the game. On my screen he didn't move, except for sort of walking on the spot. On his side he was freely playing the game too.

I've also noticed that if I move or consume and item, and then log out and come back, once I spawn in that item will be moved or consumed, but for as long as I stay in a server it will appear in my inventory.

I've updated the game at every prompt, and I'm not running experimental updates. I've been joining full servers at the top of the browser list. It seems unlikely they're running the wrong versions.